Recently I have had many questions by phone, email, and Linked-In pertaining to mold filling for open-loop molding machines. As a result, I wish to clarify the basic difference between the optimal filling strategy for closed-loop molding machines vs. open-loop molding machines with respect to compensating for viscosity variations to maintain process consistency…
1st Stage Fill
Closed-Loop Molding Machines: 1st stage injection on these molding machines is based on maintaining a consistent screw velocity. If there is adequate injection pressure available, and the machine is operating properly, the polymer will leave the barrel with a consistent shear rate. If the injection speed is high enough to cause shear thinning, the volume that is injected should be consistent over a long period of time. Although there will be normal variations in material viscosity, a process using 95% fill during 1st stage injection should maintain a short shot over an extended period of time.
Open-Loop Molding Machines: 1st stage injection on these machines is based on maintaining a consistent injection pressure. In this case, the screw velocity will drop as the resistance to flow increases… resulting in a constantly decreasing shear rate. Since this machine is incapable of maintaining a consistent shear rate, viscosity shifts will significantly affect the fill time as well as the volume of material which is injected. For this reason, an open-loop machine cannot maintain a consistent short shot during 1st stage. As a result, the most consistent processes, on open-loop machines, tend to result from completely filling and packing the part during 1st stage injection.
It is completely possible to mold acceptable parts using a complete 1st stage fill with a closed-loop machine, or a 1st stage short shot with an open-loop machine… it’s just that the process will require significantly more adjustment as the viscosity of the material shifts over time. The above comments is strictly focused on what 1st stage injection strategy generally provides the least process variation.